The detailed calorimetric analysis of the thermodynamics of the allosteric enzyme aspartate transcarbamylase will be continued. Both the native enzyme and chemically modified or mutant enzymes will be used to investigate the energetics of the interactions between the subunits and of the binding of substrates, inhibitors, and activators. The purpose of these measurements is to: (a) establish thermodynamic criteria against which models of the catalytic and regulatory mchanisms may be tested, (b) evaluate the magnitude of the various conformational changes which the enzyme undergoes and (c) elucidate the nature of the forces involved in binding small molecules to the enzyme and in subunit interactions. An analysis of the thermodynamic properties of erythrocyte and platelet membranes will be begun. Reaction calorimetry will be used to study the binding of ligands, differential scanning calorimetry to characterize thermal transitions. These measurements will contribute to our understanding of lipid-protein interactions and cooperative structural changes in membranes. The following reactions will be examined: 1. Erythrocyte ghosts: Binding of (a) Na ion, K ion, and Ca ions, (b) Plant lectins, (c) Prostaglandins, (d) Glyceraldehyde-3- phosphate dehydrogenase, aldolase and spectrin (to depleted ghosts), (e) Thermal transitions, before and after treatment with hydrolytic enzymes, and in the presence and absence of Ca ions and prostaglandins. 2. Platelet membrane vesicles: Binding and effect on thermal transitions of (a) Ca ions and Mg ions, (b) AMP, ADP, (c) Collagen and oligosaccharides, (d) Serotonin, epinephrine, histamine, (e) Prostaglandins, and (f) Plant lectins.